Does "@INC contains: CODE(0xcb9..." suggest that @INC is being corrupted when the exe fires up?

No. It's a way to have smart libraries: by including a callback (a code ref) in @INC. See require:

You can also insert hooks into the import facility, by putting directly Perl code into the @INC array. There are three forms of hooks: subroutine references, array references and blessed objects.

(You can read the rest there.)

What happens, is IMO that pp didn't know that Prima::Buttons is needed, so it didn't include it. Probably, that happens because the module gets required dynamically, I guess it could likely happen with DBD::* modules for DBI, too.

See the docs for pp, command line option -M:

% pp -M Foo::Bar hello      # Extra modules in the include path
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